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How to improve PSRR for this kind of bandgap?

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anhnha

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Hi,
I am using this kind of bandgap for my LDO.
106686d1403749108-bandgap.jpg

The problem is that PSRR is very bad. It is only -40dB at DC. I need about -80dB at DC.
Could anyone suggest some methods to improve PSRR?
My op amp has 94dB gain so I think that is not the op amp problem.
Thanks.
 

A usual first step would be to analyze where in the circuit the PS influence is generated. As a first guess, I would suspect MOSFET output impedance, but you should measure the cause in your simulation.

Text book circuits often show cascaded current sources.
 
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    anhnha

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Thank you.
How should I analyse the cause?
Could you show me how to connect cascaded current sources?
I can't find the cascaded structure for this kind of bandgap.
 

Also compensate the opamp using cascode compensation technique instead of simple cuting bandwith by capacitor connected to ground/vdd could improve your psrr.
 
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    anhnha

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Thank you.
Did you mean that cascode compensation doesn't decrease the bandwidth?
 

Of course cascode compensation decrease the bandwidth, but the psrr is better than in classic miller method.
 
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    anhnha

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Thanks. I am using folded cascode amplifier without any compensation.
I think I need to increase gain bandwidth of the amplifier.
 

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